The present invention relates to novel immunogenic compositions which can be employed in a method for eliminating, or at least substantially reducing, the offensive odor associated with uncastrated male pigs and the preparation of meats derived therefrom. These novel immunogens can be characterized as chemical conjugates of certain C.sub.19 .DELTA..sup.16 -steroids bound to a carrier protein and mixtures thereof.
The boar pig possesses a strong characteristic perspiration-like or urine-like odor, called "boar odor" or "boar taint," which is given off particularly upon heating and cooking of the meat. The odor is highly objectionable to consumers and, therefore, carcasses tainted by boar odor are either condemned or subject to restricted usage by USDA meat inspectors.
Since approximately 65% of all boars produce meat which is characterized by this offensive odor, a considerable economic disadvantage is suffered by the swine industry. As elucidated below, the only successful method of preventing boar odor, heretofore, was by castration of young male pigs. However, numerous recent studies on the effects of castration of male pigs which are raised for meat production have clearly shown that rearing uncastrated male pigs presents great economic advantages to the pork producer. Castration is not only a distasteful and labor-intensive chore which retards growth and involves the risk of infection, but it also produces animals with inferior carcass characteristics and a lower feed conversion efficiency (pounds of feed per pounds of weight gain). Rearing young boars to market weights could improve the overall efficiency of lean meat production over castrates by 25-30%.
It is widely accepted that "boar taint" is caused predominantly by the C.sub.19 .DELTA..sup.16 -steroid, 5.alpha.-androst-16-en-3-one (5.alpha.-androstenone). The steroid is synthesized in the Leydig cells of the testes and is delivered to the blood where it is taken to the adipose tissue and stored in the fat. When needed for sexual stimulation of sows, androstenone is mobilized from the fat and transported by the blood to the salivary glands where it is converted to the reduced alcohol, 5.alpha.-androst-16-en-3.alpha.-ol. Due to its lypophilic properties, 5.alpha.-androstenone and other C.sub.19 .DELTA..sup.16 -steroids are stored in fat at concentrations reaching quantities of micrograms/g of fat. At these concentrations, it is detectable by humans with consumer acceptance depending on the removal of the responsible steroids.
It is also known that the odor associated with these steroids plays an important role in the reproductive behavior of the animal by stimulating sows to assume the mating stance. Before mating, the C.sub.19 .DELTA..sup.16 -steroids are present in the boar's saliva and are transmitted to the female animal, i.e., acting as a pheromone.
Boar taint was investigated for the first time in 1936. Thirty years later, by using the mass spectrometer, R. L. S. Patterson was able to isolate a suspected steroid as the cause of boar taint. Since then, considerable resources have been dedicated to the investigation of the so-called C.sub.19 .DELTA..sup.16 -steroids and their biosynthetic pathways in an effort to eliminate boar taint.
For example, the recent publication by Shenoy, et al. ("The `boar taint` steroid 5.alpha.-androst-16-en-3-one: an immunisation trial," Acta Endocrinologica, Vol. 100, pp. 131-136 [1982]) describes an attempt to eliminate boar taint by immunizing uncastrated boars with 5.alpha.-androst-16-en-3-one (androstenone) conjugated with bovine thyroglobulin as a carrier protein. This immunization regime did not meet with success since the antibody-bound steroid complex was not removed from the body, and, as it was postulated, having not been recognized by the animal's "feedback" mechanism, actually increased the production of the steroid in the animal.
Further work was done by Williamson and Patterson ("A selective immunization procedure against 5.alpha.-androstenone in boars," Anim. Prod., Vol. 35, pp. 353-360 [1982]) wherein a similar procedure selective for boar 5.alpha.-androstenone is described. Auto-immunization was effected by the administration of a conjugate of the steroid and bovine serum albumin (BSA). According to these studies, the specific antibody formed sequesters and removes androstenone as it is secreted into the peripheral circulation, thus effectively preventing its transfer to the adipose tissue. However, no results relative to the organoleptic characteristics of the meat from the immunized boars were reported for this protocol.
Earlier, Claus, in the publication entitled, "Neutralization of Pheromones by Antisera in Pigs," Immunization with Hormones in Reproduction Research, ed. E. Nieschlag, North-Holland Publishing Co. (1975) described a method of immunizing boars with 3-carboxymethyloximeandrostenone conjugate bound to BSA as a carrier protein. The results, although "encouraging," did not yield a completely satisfactory method for eliminating the taint in boar carcasses.
U.S. Pat. No. 4,384,206 to Bjarno discloses a process for the statistical detection of boar odor by the determination and evaluation of comparative IR-spectrophotometrical transmission data. The reference fails to describe an immunization regime in any manner.
Although methods of immunization to eliminate boar taint are shown by the prior art, each of these methods employ an immunogen only of 5.alpha.-androst-16-en-3-one conjugated with a carrier protein. Each of these regimes requires continuous administration and does not effectively eliminate boar taint nor the concomitant accumulation of the boar taint steroid.